Mars's Southern Interior Is Hundreds of Degrees Hotter Than Its North, Gravity Data Show

Researchers have found that Mars's interior is far from uniform in temperature — the region beneath the planet's southern hemisphere is about 200 to 400 degrees Celsius hotter than the north, and part of it may even be partially molten, according to a study published August 27 in the journal Nature.

The study was led by Alexander Berne, a Caltech PhD graduate (class of 2026) who is now a postdoctoral associate at the University of Arizona. While at Caltech, Berne developed a technique called tidal tomography, which uses small changes in a planet's gravity over time to map the structure hidden inside it.

To build the picture of Mars's interior, Berne and his colleagues analyzed decades of tracking data from three NASA missions — Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter. Because Mars follows a slightly elliptical orbit and spins on a tilted axis, the Sun's gravitational pull on it changes with the seasons. By measuring how this shifts the tiny velocities of the orbiting spacecraft, the team reconstructed Mars's gravitational field in three dimensions.

Mars's two hemispheres already look very different on the surface: the south is covered in tall mountains and heavily cratered terrain, while the north consists mostly of broad, low-lying plains. The new gravity data show that this divide runs deep, with the southern interior far hotter than the north.

The heat difference could help explain two other Martian puzzles. Unusual magnetic signatures found in iron-rich minerals in the south could be explained if a hotter southern mantle once supported a strong magnetic field there. And an observation from NASA's InSight lander that seismic waves lose energy faster in the south could also be explained by higher underground temperatures.

Co-author Amirhossein Bagheri, a Caltech postdoctoral scholar and former member of the InSight team, said the north-south difference is important because it gives clues about processes that shaped Mars's hydrology, including the formation of basins that may once have held water.

The researchers have not yet determined what caused the thermal anomaly. They are considering three possibilities: a giant impact that released heat from the north, spontaneous convection that once occurred inside the southern mantle, or thick geological structures in the south that trapped heat and kept it from escaping. The study, titled "Tidal Tomography Reveals a Thermal Anomaly Beneath Mars's Crustal Dichotomy," was funded by NASA.